1. Field of the Invention
The invention concerns suppression of electrical noise in motors which are controlled by mechanical relays and, in particular, motors in which the relays periodically reverse the current fed to the motors, in order to reverse motor rotation. Windshield wipers in motor vehicles utilize such motors.
2. Description of Related Art
FIG. 1A illustrates a prior-art system for changing direction of an electric motor M, which drives windshield wipers (not shown) in an automotive vehicle. When the relays S1 and S2 are in the position shown in FIG. 1B, current I flows in the direction indicated, and the motor M rotates in one direction. When the relays S1 and S2 switch to the position shown in FIG. 1C, the current reverses to Ir, and the motor reverses direction.
This motor M is can be located on a tailgate of a motor vehicle.
The Inventors have found that the system of FIGS. 1A-1C produces the radio-frequency emissions shown in FIGS. 3A-3B. The dotted line in FIG. 3 indicates a reference, for use in comparison with other plots, discussed later.
These emissions can interfere with radios, tape players, televisions, ham radios, cellular telephones, and possibly radar detectors, which are used by the occupants of the vehicle, or used by nearby persons. Capacitors Cl and C4 in FIGS. 1A-1C are intended to suppress these emissions, but, as the plot of FIGS. 3A-3B indicate, they do not achieve full suppression.